In the manufacture of ophthalmic lenses, a thermoplastic, such as a polycarbonate resin, is typically injected through a gate to a molding cavity defined between two compressible mold blocks that support mold dies having optically finished surfaces to be impressed on the injected thermoplastic. A discussion of the production of ophthalmic lenses by this conventional process can be found in U.S. Pat. No. 4,900,242 to Maus et al., which is incorporated by reference herein.
Glass is a preferred material for constructing lens dies used to produce ophthalmic lenses. Glass lens dies can be finely polished and therefore produce ophthalmic lenses with superior optical properties.
Glass lens dies also have superior insulating capabilities and maintain a more stable heating environment and more efficiently maintain the temperature in the mold cavity during the lens formation. In the higher temperature cavity, the formed lens has a thinner skin and correspondingly reduced surface stresses.
Conventional techniques for securing lens dies to their associated supports are not well suited for securing relatively fragile lens dies made of glass. The instances of breakage using known lens die mounting systems result in increased operating costs and in some circumstances render use of glass lens dies economically infeasible.
A glass lens die can be held against a mold block by a surrounding retaining ring or sleeve, which is, in turn, secured to the mold block. However, this mounting system requires appreciable time to remove and replace the lens die.
It is desirable to interchange different lens dies during lens production to provide lenses of different powers and other optical characteristics. The down time associated with such changeovers is typically on the order of several minutes and sometimes hours and can typically only be cost-justified in lens manufacture when a significant quantity of lenses have been produced using a mounted lens die before changeover occurs.
Adhesives present an alternative for mounting glass dies to underlying supports. However, the unpredictable and relatively uncontrollable variation in the adhesive thickness along the mounting interface creates points of leverage and stresses that often break glass lens dies. The adhesives also make the underlying support difficult to reuse with other lens dies, making frequent changeover difficult.